1. Field of the Invention
This invention relates to a power steering and limited slip differential system suitable for vehicles, in particular, for trucks and buses.
2. Description of the Related Art
Generally, phase lag and gain of a vehicle's handling response increases with vehicle speed. Excessive phase lag will cause delay in the vehicle reaching the driver's intended course, resulting in oversteering, while excessive gain will intensify the oversteering, making the vehicle weave.
Further, high phase lag will also increase the time required for disturbance, such as road surface roughness, to be transmitted to the driver's hand on the steering wheel as a response, delaying corrective steering and producing steering wheel weave. In the case of trucks and buses, the phase lag magnitude in particular can reach four or five times that of passenger cars.
Still further, expressways in the middle of the night are analogous to huge belt conveyers filled up with groups of trucks. These trucks are flowing towards the metropolitan Tokyo and arrive at wholesale markets etc. before dawn. A between car distance in these truck groups is generally short and the speed thereof is high. Further, highway bus services between cities are spotlighted. Because of their advantages such as inexpensive fares and ease of use, living space with a high sense and high quality, and attentive services, not only the night bus services which were understood at the beginning to supplement railway train services, but also day time highway bus services are increasing. As such, on one hand, the demand for high speed and long distance services by the trucks and buses is increasing, however, on the other hand, in particular, with regard to trucks, a shortage of truck drivers is serious, and in addition, age of the drivers has been increasing.
For these reasons, there appears an indication of woman driver expansion.
Further, the higher the speed of a vehicle is, the more the response performance thereof reduces. Still further, according to W. Woodson and D. Conover, Human Engineering Guide for Equipment Designers 6-11, 6-20 (1973), physiological abilities relating to driving skill of advanced age drivers and women drivers are relatively low in comparison with that of young men (see FIG. 1).
Therefore, a system is desired which realizes a compensation for the decrease in the response performance at high speed region and the physiological ability differences of the drivers within the vehicle. Moreover, large sized vehicles which necessitate relatively wide space on the running road in comparison with passenger cars have to be provided with an even better response performance than passenger cars.
Further, with regard to desirable vehicle response characteristics, a vehicle's controllability and stability have to be investigated from both the vehicle response performance in association with driver's handling and the vehicle response performance in association with disturbances such as those caused by roughness of the road surface.
First, with regard to the handling response characteristic, a factor T.beta. is defined as the product of the time constant and yaw gain. The smaller the factor is, the higher is the subjective judgment of drivers. There is an optimum region of course tracking characteristic in a range of small time constants and of certain amounts of yaw acceleration gain.
These facts concern passenger cars, however, these passenger car tendencies are similar in trucks and buses.
FIG. 2 shows an example of the response characteristic of a truck and bus.
The truck is equipped with a front engine and leaf suspensions, while the bus is equipped with a rear engine and air suspensions, and the gain and phase lag of the truck are smaller than those of the bus. Further, the subjective judgment from the driver's viewpoint of the truck is better than the bus.
These gain and phase lag increase in response to an increase of the vehicle speed and thereby the burdens to drivers increase. This suggests that a desirable handling response region will be achieved by reducing both the gain and phase lag smaller than those now.
This tendency meets with the previous tendency with regard to the passenger car data, when the phase lag is assumed to belong to the time constant property. When the phase lag is large, approach to an aimed course by a driver is delayed so that an oversteering is caused, and when the gain is much larger, the oversteering is amplified so that the vehicle weaves. Still further, the problems arising from the physiological ability differences can be compensated for by reducing the time constant in the steering system.
Nextly, with regard to disturbance response characteristics, it is desirable to reduce the effects caused by disturbances as much as possible. For example, the irregularity of the road surface causes displacement of the axles and the chassis in succession. The displacement is sensed by the driver, and thereafter the driver's corrective steering begins. A smaller delay until the corrective steering and greater damping of the disturbances are desirable. However, if the disturbances can be intercepted at the onset so as not to permit their entry, such is considered the best way.
Further, with regard to the handling response, when the handling response performance of trucks and buses is compared with that of passenger cars, there is a significant difference in connection with the phase lag, which is very large in trucks and buses (see FIG. 3). For analyzing what causes the phase lag to be so large, the time lag from the initiation of steering operation to the beginning of the course change of the vehicle was measured along its transfer route (see FIGS. 4 and 7).
As the result, it was found that the time lag in the steering system occupies 50% of the total time lag. Further, with regard to the chassis system, the time lag of buses is larger than that of trucks, the reasons of which are considered to be due to influences such as their suspension structure differences and weight allotment differences to the front and rear axles.
These trucks and buses employ common steering systems so that, with regard to the time lag in their steering systems, there is no difference.
Further, several studies reported to have shortened the time lag in the steering system which amounts to 60% among the total time lag. Some of these went further to suggest specific system structures but failed to reach practical uses.
Two reasons are presumed for the failures. One is that it is presumed that the phase lag was shortened, however concurrently the gain gets large, and thereby the steering wheel becomes sensitive and the handling feeling is deteriorated. The other is that it is presumed that since the mechanical coupling between the steering wheel and the front wheels has been disconnected, problems in connection with safety are unsolved.
In view of the above, such measures are required that reduce phase lag without increasing the time lag and without disconnecting the mechanical coupling.
On the other hand, with regard to the disturbance response, as indicated above, the disturbances should be intercepted at the onset. In other words, it is desirable to intercept the entering of disturbance at the tires from which the disturbances intrude. The countermeasures thereto are the compliance steering control of the rear wheels and the torque split control between the right and left rear wheels and, in particular, in case of the rigid rear axle structure with leaf springs which is employed in many large commercial vehicles, the torque split control is preferable.